|Year : 2013 | Volume
| Issue : 1 | Page : 14-17
Laryngeal movements in stutterers
Swapna Sebastian1, Anto Suresh Benedict2, Achamma Balraj1
1 Department of Ear Nose and Throat, Christian Medical College, Vellore, Tamil Nadu, India
2 Natlab, Statue, Trivandrum, Kerala, India
|Date of Web Publication||24-Sep-2013|
Department of ENT, Christian Medical College, Vellore 632004, Tamil Nadu
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Background: The etiology of stuttering is a puzzle. Abnormal laryngeal movements have been assumed to be a cause of stuttering by many researchers. We aimed at comparing the laryngeal functions of stutterers using electroglottogram (EGG) and vocal tract functioning using formant frequency analysis. Materials and Methods: We compared the acoustic and electroglottographic parameters in the phonation of stutterers between the age range of 15-25 years and normal subjects matched for age and sex. Results and Discussion: All the electroglottographic parameters of stutterers differed from that of normal speaking subjects which is indicative of difficulty in adjustment of the laryngeal gestures for speech in stutterers. There was no significant difference between the stutterers and non-stutterers in terms of formant frequencies. Similar formant frequencies in stutterers and normal speakers and abnormal electroglottographic parameters are suggestive of abnormal laryngeal movements and normal vocal tract functioning in stutterers.
Keywords: Laryngeal behavior, stutterers, acoustic analysis, electroglottogram
|How to cite this article:|
Sebastian S, Benedict AS, Balraj A. Laryngeal movements in stutterers. J Laryngol Voice 2013;3:14-7
| Introduction|| |
Stuttering is a deviation in the ongoing fluency of speech, and an inability to maintain the connected rhythm of speech. It is temporal disruption of simultaneous and successive programming of muscular movements required to produce a speech sound or its link to the next sound.  No definite cause for stuttering has been identified. Larynx has been assumed to be the root cause by many researchers. Laryngeal behavior in stutterers has been an area of interest to many investigators. ,,, Inappropriate laryngeal gestures ,,, and inappropriate vocal fold positioning  have been reported by many researchers.
Schaferskupper and Simon  have stated that corresponding to the higher tension of muscles involved in speech production, a higher mean fundamental frequency should be expected in stutterers as compared to nonstutterers.
Schwartz  believed that stuttering is due to inappropriate vigorous contraction of the posterior cricoarytenoid muscle in response to subglottal air pressures required for speech.
Extensive study on the laryngeal muscle activity during speech , revealed higher levels of muscle activity in stutterers which dropped suddenly when a stuttered word was finally uttered, disruption of coordinated muscle activity of the abductor and the adductor (in normal speaker these muscle acts with reciprocity, that is, when the abductor contracts the adductor relaxes and vice versa.) and abnormal activity of cricothyroid muscle, posterior cricoarytenoid, and lateral cricoarytenoid.
Perkins et al.,  hypothesized that stuttering results from the speakers' difficulty to coordinate phonation with articulation and respiration.
Voice onset time has been defined as the time elapse of consonant burst to the onset of periodic glottal vibration for the production of vowel that follows the consonant. Spectrographic study on 23 stutterers by Agnello  revealed that voice onset time and voice termination time in stutterers were slower than non-stutterers. Spectrographic analysis of consonant vowel syllables taken from oral reading of 12 child stutterers and matched group of non-stutterers by Hillman and Gilbert  revealed longer voice onset time in stutterers in the perceptually fluent utterances. Hoit et al.,  found voice onset time to be longer at high lung volumes. Stutterers have been reported to be slower than normal in initiating voicing during reaction time experiments even during the fluent utterances ,, (Adams and Hayden, 1976; Cross and Luper 1979; Starkweather, Hirschman, and Tannenbaum, 1976).
Based on electroglottogram (EGG) study, Borden et al.,  reported the vocal cords of stutterers to be stiff, which was supported by an increase in fundamental frequency. Based on the studies on EGGand acoustic analysis, Chevrie-Muller  (1963) reported irregularities in terms of hard glottal attack, delayed transitions, and clonic fluttering of vocal folds.
The purpose of our study was to compare the acoustic and electroglottographic parameters in the phonation of stutterers and normal subjects matched for age and sex.
| Materials and Methods|| |
Ten male stutterers between the age range of 15-25 years and ten non-stutterers (controls) matched by sex and age served as subjects. The severity of the stutterers varied from moderate to severe, according to the stuttering severity index  [Table 1].
The subjects were seated comfortably in a sound treated room and were tested individually. Electroglottograph (Kay Elementrics Corporation) was used for EGG recording which was connected to a speech interface unit and then to a personal computer to obtain the display of the glottal waveform. Two electrodes were placed on the neck thyroid alae. The positions of the electrodes were adjusted until clear laryngeal waveform appeared on the display screen when the subjects phonated. The subjects were instructed to phonate the vowel 'a' for 5 sec at their natural and comfortable loudness and pitch levels. The following parameters were derived from electroglottographic recordings:
- Opening time: Time taken for vocal cords to come from the medial position to lateral position
- Open time: It is the duration during which the vocal cords stay in the open position (lateral position)
- Closing time: Time taken for the vocal cords to come from the lateral position to medial position
- Close time: The duration during which the vocal cords stay in the medial position (closed position)
For acoustic analysis, the speech stimuli were recorded in a sound treated room with an ambient noise below 40 dB on high-bias metal cassettes, using a professional stereo cassette desk (Ahuja) and a AKG-D222 dynamic cardioid microphone with a flat frequency response from 50-15,000 Hz. The microphone to mouth distance was approximately 10 cm for all the subjects. All the instruments were calibrated prior to the experiments as per the instructions given in the manual of instruments.
The recorded speech samples were digitized at the rate of 8k Hz using 12 bits and spectrographic analysis was done to find the formant frequencies using the Speech Science Lab (SSL) software developed by Voice Speech Systems, Bangalore. Formant frequencies are acoustic resonances of the human vocal tract. F1, F2, and F3 are the midpoint of visible dark bands of energy appropriate to the first three vowel resonances. The formant with the lowest frequency is called f1, the second f2, and the third f3.
| Results|| |
Comparison of the electroglottographic parameters of stutterers with normal speakers was done using paired t-test [Table 2]. All the electroglottographic parameters of the speech of stutterers differed significantly when compared to non-stutterers. Acoustic analysis was done to compare the formant frequencies of stutterers with normal speakers. There was no significant difference between the stutterers and non-stutterers in terms of formant frequencies [Table 3].
|Table 2: The results of paired t test to find the difference between stutterers and normal subjects for different EGG parameters of voice |
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|Table 3: The results of paired t-test to find the difference between stutterers and normal subjects for the formant frequencies of voice |
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| Discussion|| |
Electroglottography is a noninvasive method of indirectly examining the degree of contact between the vocal folds. High frequency current is passed between two electrodes placed on each side of the thyroid prominence and measures the impedance of laryngeal tissues across time.  (Fourcin 1974). The impedance to the transmission of current decreases as the contact between the vocal fold increases. The resistance of human tissue to the flow of electrical current is much less than the resistance air has. When the glottis is closed, there is much less impedance than when the glottis is open (and filled with air), and so the flow of current increases. Thus, the opening and closing of the vocal folds are inferred from the changes in signal amplitude.
All the electroglottographic parameters of the speech of stutterers differed significantly when compared to nonstutterers. Opening time has been defined as the time for the vocal cords to come from medial to lateral position. There was a significant difference between normal speakers (mean = 3.47 ms) and stutterers (mean = 1.63 ms) in terms of opening time. The opening time was significantly less for the stutterers compared to normal speaking group. This indicates that the vocal cords opened abruptly in stutterers compared to normal group. Open time is the duration during which the vocal cords stay in the open position in a cycle of vocal fold movement. Stutterers had significantly less open time (mean = 0.71 ms), that is, the vocal cords closed abruptly compared to normal speakers (mean = 1.47 ms).
Closing time is the time taken for vocal cords to come from lateral to medial position. Stutterers took significantly more closing time (mean 3.73 ms) compared to normal speakers (1.96 ms).
Close time has been defined as the duration during which the vocal cords stay in the medial position (closed position). Stutterers (mean = 1.31 ms) differed significantly from normal group (mean = 0.77 ms) in terms of close time (0.043 ms). They had more close time, that is, the vocal cords stayed in the medial position for longer time when compared to normal speaking group.
The stuttering group (mean = 210.8) showed significantly higher fundamental frequency compared to age and sex matched controls (mean = 127.4 Hz). The higher fundamental frequency in stutterers could be attributed to the increase in tension of laryngeal muscles especially the cricothyroid muscles. Schaferskupper and Simon  have also reported a higher mean fundamental frequency in stutterers as compared to nonstutterers.
The above abnormalities in the EGG parameters are indicativeof difficulty in the adjustment of laryngeal gestures for speech in stutterers.
There was no significant difference between the stutterers and nonstutterers in terms of formant frequencies which is suggestive of normal vocal tract functioning. Formant frequencies reflect the characteristic filter function of the vocal tract based on its size and shape. The sound signal created in the larynx passes through the supralaryngeal vocal tract before being radiated into the environment. The column of air in the vocal tract has certain natural modes of vibration or resonances, which affect the resultant output signal  (Fant, 1960) which is called ''formants''. The length and shape of the vocal tract are the main determinants of formant frequencies of speech sounds. Similar formant frequencies in stutterers and normal speakers and abnormal EGG parameters are suggestive of abnormal laryngeal movements and normal vocal tract functioning in stutterers.
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[Table 1], [Table 2], [Table 3]